A thin-film transistor (TFT) is a kind of field effect transistor (FET). A TFT is basically a three-terminal element that has a gate terminal, a source terminal, and a drain terminal, and is an active element having a function of using a semiconductor thin film that is formed on a substrate as a channel layer in which electrons or holes migrate, applying a voltage to the gate terminal, controlling the current that flows in the channel layer, and switching the current between the source terminal and the drain terminal. Currently, polycrystalline silicon film or amorphous silicon film is widely used as the channel layer of a TFT.
The amorphous silicon film is capable of being uniformly formed on a 10th-generation glass substrate having a large surface area, and is widely used for the channel layer of a liquid-crystal panel TFT. However, the mobility of carrier electrons (carrier mobility) is a low 1 cm2/Vsec or less, so application in a high-definition panel TFT is becoming difficult. In other words, as liquid crystals become to have higher definition, there is a need for high-speed driving of a TFT, and in order to achieve high-speed driving of this kind of TFT, it is necessary to use a semiconductor thin film having a carrier mobility that is higher than the 1 cm2/Vsec carrier mobility of an amorphous film as the channel layer.
On the other hand, the polycrystalline silicon film has a high carrier mobility of about 100 cm2/Vsec, so has sufficient characteristics as channel-layer material for a high-definition panel TFT. However, a polycrystalline silicon film has low carrier mobility at the crystal grain boundaries, so there is a problem in that there is poor uniformity in the surface of the substrate, and variation in the TFT characteristics occurs. Moreover, in the manufacturing process of a polycrystalline silicon film, after forming an amorphous film at a comparatively low temperature of 300° C. or lower, the film is crystallized by an annealing process. This annealing process is a special process that employs excimer laser annealing or the like, so a high running cost is required. In addition, the size of the glass substrate that can be used remains at a 5th-generation size, there is a limit to the reduction in cost, and product development is also limited.
Therefore, currently as material for the channel layer of a TFT, there is a need for a material that has both the excellent characteristics of an amorphous silicon film and a polycrystalline silicon film and that can be obtained a low cost. For example, JP 2010-219538 (A) discloses a transparent amorphous oxide thin film (a-IGZO film) that is formed by a vapor phase film formation method, has In (indium), Ga (gallium), An (zinc), and O (oxygen) without added impurity ions, and has a carrier mobility that is higher than 1 cm2/Vsec, and a carrier density that is 1016/cm3 or less.
However, even though the a-IGZO film disclosed in JP 2010-219538 (A) that is formed by a vapor phase film formation method such as a sputtering method or pulse laser vapor deposition method has a comparatively high carrier mobility within the range 1 cm2/Vsec to 10 cm2/Vsec, an amorphous oxide thin film inherently has problems in that oxygen deficiency easily occurs, and operation of a device such as a TFT sometimes becomes unstable because of the fact that the behavior of the carrier electrons is not always stable due to external factors such as heat.
Furthermore, the occurrence of a phenomenon in which the threshold voltage will shift to the negative side when a negative bias is continuously applied to a TFT element under visible-light irradiation (light negative bias degradation phenomenon), which is unique to amorphous film, is identified as a serious problem for use in displays such as a liquid-crystal display.
On the other hand, JP 2008-192721 (A) discloses an In2O3 (indium oxide) film that is doped with Sn (tin), Ti (titanium) or W (tungsten), or a In2O3 film that is doped with W, Zn and/or Sn with the object of obtaining a thin-film transistor that is low cost and is able to achieve both high performance and high reliability, and that can be used for manufacturing elements for a polymeric material that does not require high-temperature processing. According to JP 2008-192721 (A), by using these amorphous In2O3 films for a channel layer, it is possible to make the carrier mobility of a TFT element 5 cm2/Vsec or greater.
Moreover, JP 2010-251604 (A) discloses an amorphous In2O3 film that is obtained by forming a film using a no-heating sputtering method with an In2O3 sintered compact that is doped with one or two or more elements selected from among Sn, Ti, W, and Zn as a target, and then performing heat treatment for 10 minutes to 120 minutes at 150° C. to 300° C. According to JP 2010-251604 (A), by performing this kind of heat treatment, it is possible to obtain a very stable In2O3 film through comparatively simple control while maintaining the characteristic of having both high carrier mobility and an amorphous nature.
Furthermore, in JP 2011-58012 (A) discloses an amorphous In2O3 film having excellent stability that includes In, Ga, Zn, O, and N (nitrogen), and is controlled so that the concentration of N is no less than 1×1020 atom/cc and no more than 1×1022 atom/cc.
However, the In2O3 films that are disclosed in the cited literature above are amorphous films, so basically problems such as the easy occurrence of oxygen deficiency, the films becoming unstable due to external factors such as heat, or the problem of the occurrence of the light negative bias degradation phenomenon cannot be solved. Moreover, in considering use as a channel-layer material for a high-definition panel TFT, achieving even higher carrier mobility is desired.
On the other hand, “Applied Physics Express 5 (2012) 011)” discloses an In2O3 film doped with Ga that is obtained by forming an amorphous film using a sputtering method with an In2O3 sintered compact doped with Ga as a target, and then crystallizing the film by performing heat treatment for 1 hour at 300° C. Even though this film has high carrier mobility, controlling oxygen deficiency is difficult, and the carrier density become a high 1×1017 cm−3, so it is difficult to obtain a TFT element having stable characteristics.
Moreover, JP 2009-275236 (A) discloses an oxynitride semiconductor thin film that includes added elements such as Zn and an added elements such as In or Ga, the atomic composition ratio of N that is represented by N/(N+0) is 7 atomic % or greater but less than 100 atomic %. This oxynitride semiconductor thin film can be formed by introducing source gas that includes N (nitrogen) in the gaseous phase, or irradiating a N radical using a radical source, then forming a film using a sputtering method or vapor deposition method, and further arbitrarily performing heat treatment at a temperature of 150° C. to 450° C. after film formation. According to JP 2009-275236 (A), this oxynitride semiconductor thin film has a hexagonal crystal structure, high carrier mobility of 10 cm2/Vsec to 30 cm2/Vsec, and excellent stability. However, the carrier density is high at about 1×1018 cm−3.